In the conventional syntheses of nucleosides using the silyl method, as described, for example in U.S. Pat. No. 3,748,320 (German Pat. No. 1,919,307) or in U.S. Pat. No. 4,082,911 (DOS No. 2,508,312), it has been universally considered absolutely necessary to silylate the nucleoside bases, especially the pyrimidine bases, such as uracil, 2-thiouracil, cytosine, etc., as well as the purine bases such as adenine, N.sup.6 -benzoyladenine, hypoxanthine, xanthine and guanine, prior to the actual nucleoside synthesis. Only thereafter are the thus-formed, moisture-sensitive persilylated nucleoside bases reacted in a second reaction step with blocked 1-halogen sugars, 1-O-alkyl sugars and especially 1-O-acyl sugars in the presence of Friedel-Crafts catalysts, such as SnCl.sub.4, TiCl.sub.4, ZnCl.sub.2, BF.sub.3 -etherate, AlCl.sub.3, SbCl.sub.5, or trimethylsilyl perfluoro alkyl sulfonates and/or (CH.sub.3).sub.3 SiClO.sub.4.
The reaction of silylated bases with blocked sugars to form nucleosides is also known from U.S. patent application Ser. No. 670,741, filed Mar. 26, 1976, now allowed and U.S. Pat. Nos. 3,708,469, 3,891,623, 3,983,104 and 4,090,021.
The need to separately silylate the nucleoside base has led to disadvantages such as lower yields of end product nucleoside and, of course, the attendant time and expense associated with the extra separate step.